In physiology, the water content of an animal body is contained in the tissues, the blood, the bones and elsewhere. Body water makes up a significant fraction of the human body, both by weight and by volume.
The usual way of adding water to a body is by drinking. In addition, water enters the body with foods, especially those rich in water, such as plants, raw meat, and fish. The amount of this water that is retained in animals is affected by several factors. For instance, animal body water amounts vary with the age of the animal. The older the vertebrate animal, the higher its relative bone mass and the lower its body water content. Water in the animal body performs a number of functions: as a solvent for transportation of nutrients; as a medium for excretion; a means for heat control; as a lubricant for joints; and for shock absorption.
Most of animal body water is contained in various body fluids. These include Intracellular fluid; Extracellular fluid; Plasma; Interstitial fluid; and Transcellular fluid. Water is also contained inside organs, in gastrointestinal, cerebrospinal, peritoneal, and ocular fluids. Adipose tissue contains about 10% of water, while muscle tissue contains about 75%.
By weight, the average human adult male is approximately 65% water. However, there can be considerable variation in body water percentage based on a number of factors like age, health, weight, and gender. In a large study of adults of all ages and both sexes, the adult human body averaged ~65% water. However, this varied substantially by age, sex, and adiposity (amount of fat in body composition). The figure for water fraction by weight in this sample was found to be 48 ±6% for females and 58 ±8% water for males.
The body water constitutes as much as 73% of the body weight of a newborn infant, whereas some obese people are as little as 45% water by weight. These figures are statistical averages, and so like all biostatistics, the estimation of body water will vary with factors such as type of population, age of people sampled, number of people sampled, and methodology. So there is not, and cannot be, a figure that is exactly the same for all people, for this or any other physiological measure.
In diseased states, where body water is affected, the compartment or compartments that have changed can give clues to the nature of the problem. Body water is regulated by hormones, including anti-diuretic hormone (ADH), aldosterone and atrial natriuretic peptide.
In Netter's Atlas of Human Physiology, body water is broken down into the following compartments:
- Intracellular fluid (2/3 of body water). Per Guyton, in a body containing 40 litres of fluid, about 25 litres is intracellular, which amounts to 62.5% (5/8), close enough to the 2/3 rule of thumb. Jackson's texts states 70% of body fluid is intracellular.
- Extracellular fluid (1/3 of body water). Per Guyton's illustration, for a 40 litre body, about 15 litres is extracellular, which amounts to 37.5% Again, this is close to the 1/3 rule of thumb cited here.
- Plasma (1/5 of extracellular fluid). Per Guyton's illustration, of the 15 litres of extracellular fluid, plasma volume averages 3 litres. This amounts to 20%, the same as per Netter's Atlas.
- Interstitial fluid (4/5 of extracellular fluid)
- Transcellular fluid (a.k.a. "third space," normally ignored in calculations)
Dilution and equilibration
Total body water can be determined using Flowing afterglow mass spectrometry measurement of deuterium abundance in breath samples from individuals. A known dose of deuterated water (Heavy water, D2O) is ingested and allowed to equilibrate within the body water. The FA-MS instrument then measures the deuterium-to-hydrogen (D:H) ratio in the exhaled breath water vapour. The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D2O ingested.
Different substances can be used to measure different fluid compartments:
- total body water: tritiated water or heavy water.
- extracellular fluid: inulin
- blood plasma: Evans blue
Intracellular fluid may then be estimated by subtracting extracellular fluid from total body water.
Bioelectrical impedance analysis
Another method of determining total body water percentage (TBW%) is via Bioelectrical Impedance Analysis (BIA). In the traditional BIA method, a person lies on a cot and spot electrodes are placed on the hands and bare feet. Electrolyte gel is applied first, and then a weak current of frequency 50 kHz is introduced. This AC waveform allows the creation of a current inside the body via the very capacitive skin without causing a DC flow or burns, and limited in the ~20mA range current for safety.
BIA has emerged as a promising technique because of its simplicity, low cost, high reproducibility and noninvasiveness. BIA prediction equations can be either generalized or population-specific, allowing this method to be potentially very accurate. Selecting the appropriate equation is important to determining the quality of the results.
For clinical purposes, scientists are developing a multi-frequency BIA method that may further improve the method's ability to predict a person's hydration level. New segmental BIA equipment that uses more electrodes may lead to more precise measurements of specific parts of the body.
Na+ loss approximately correlates with fluid loss from extracellular fluid (ECF), since Na+ has a much higher concentration in ECF than intracellular fluid (ICF). In contrast, K+ has a much higher concentration in ICF than ECF, and therefore its loss rather correlates with fluid loss from ICF, since K+ loss from ECF causes the K+ in ICF to diffuse out of the cells, dragging water with it by osmosis.
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- Guyton, Arthur C. (1991). Textbook of Medical Physiology (8th ed.). Philadelphia: W.B. Saunders. p. 274. ISBN 0-7216-3994-1.
- See table 1. here
- Guyton, Arthur C. (1991) p. 275
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- Body water at the US National Library of Medicine Medical Subject Headings (MeSH)
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